1. Field of the Invention
The present invention relates generally to ink-jet hard copy apparatus and, more specifically, to methods and apparatus for drying ink deposited on print media during real-time printing operations.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 (PRIOR ART) depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller 102 (usually a microprocessor or application specific integrated circuit (xe2x80x9cASICxe2x80x9d) controlled printed circuit board) connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC""s. Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown, but see FIG. 2) to an internal printing station, or xe2x80x9csprint zone,xe2x80x9d where graphical images or alphanumeric text is created. A carriage 109, mounted on a slider 111, scans the print medium. An encoder 113 is provided for keeping track of the position of the carriage 109 at any given time. A set 115 of individual ink-jet pens, or print cartridges, 117A-117D are releasable mounted in the carriage 109 for easy access (generally, in a full color system, inks for the subtractive primary colors, cyan, yellow, magenta (CYM) and true black (K) are provided). Once a printed page is completed, the print medium is ejected onto an output tray 119. It is common in the art to refer to the pen scanning direction as the x-axis, the paper feed direction as the y-axis, and the ink drop firing direction as the z-axis.
In essence, the ink-jet printing process involves dot-matrix manipulation of droplets of ink ejected from a pen onto an adjacent print medium (for convenience of explanation, the word xe2x80x9cpaperxe2x80x9d is used hereinafter as generic for all forms of print media regardless of its individual constitution). An ink-jet pen includes a printhead which consists of a number of columns of ink nozzles. A column of nozzles (typically less than one-inch in total height) selectively fires ink droplets to create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned across the media. A given nozzle of the printhead is used to address a given vertical print column position, referred to as a picture element, or xe2x80x9cpixel,xe2x80x9d on the paper. Horizontal positions on the paper are addressed by repeatedly firing a given nozzle as the pen is scanned. Thus, a single sweep scan of the pen can print a swath of dots. The paper is stepped to permit a series of contiguous or overlapping swaths. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. In the state of the art, the fired droplets of ink are measured in picoliters in volume, producing a printed dot of only about {fraction (1/600)}th inch in diameter; high-end commercial printers are know to produce a 1200 DPI (dots per inch) image.
An important factor in printing with wet ink drops is drying time. The printing of high density plots on plain paper suffers two major drawbacks. First, the saturated media is transformed into an unacceptably wavy sheet. xe2x80x9cInkxe2x80x9d generally can be dye-based or pigment-based and uses water or another evaporative solvent as a carrier. When an image to be recorded has high density, a large amount of water is applied to and driven into the medium which in turn swells erratically, causing the printed regions to become wavy or wrinkled, a phenomenon generally known as xe2x80x9ccockling.xe2x80x9d Secondly, adjacent colors tend to run, or xe2x80x9cbleed,xe2x80x9d into one another. Both phenomena degrade print quality.
Preheating the media and post print zone heating of the media are both known in the prior art. In order to speed ink dot drying time on the paper surface and reduce or eliminate cockle and bleed, the print zone is sometimes heated concurrently with the printing operation. In U.S. Pat. No. 5,287,123 for a PREHEAT ROLLER FOR THERMAL INK-JET PRINTER, MEDIN (common inventor herein) et al. (hereinafter referred to as Medin ""123) disclose a heating blower system for evaporating ink carriers from the print medium during real-time ink-jet printing. As illustrated summarily in FIG. 2 (PRIOR ART), and referring simultaneously to FIG. 1, Medin ""123 provided a cross-flow fan 201 at the exit side of a print zone subjacent a pen 117. The cross-flow fan directs an air flow, arrows 203, at a sheet 205 of print media 105 through the print zone (see xe2x80x9cMEDIA DIRECTIONxe2x80x9d labeled arrows 211) in order to cause turbulence at the medium surface being printed to thereby accelerate evaporation. An exhaust fan 207 having a duct system 209 exhausts air and ink carrier vapor away from the print zone and out of the printer. While in the Hewlett-Packard(trademark) PaintJet(trademark) printer model XL300 contemporary of the patented Medin device, ink drops ranged from forty to one-hundred twenty picoliters in volume, in the current state of the art drop volume has been reduced to ten picoliters. Thus, the ink droplets are much more susceptible to being affected by a cross-flow fan.
There is a need for improved methods and apparatus for scrubbing print media surface boundary layers and for preventing airstreams in the print zone from affecting ink drop flight between pen and paper, while still decreasing cockle and bleed problems inherent in ink-jet printing by improving ink dot drying time.
In its basic aspects, the present invention provides an ink-jet hard copy apparatus for printing onto a print media, including: ink-jet mechanisms for selectively printing dots of ink on an adjacently positioned print medium at a print zone of the apparatus; transport mechanisms for advancing the print medium via a print medium path through the print zone; and disposed within the apparatus proximate the print zone, airflow mechanisms for producing a substantially laminar flow of air through the print zone during printing operations.
In another basic aspect, the present invention provides a method for drying ink drops deposited on print medium by an ink-jet writing mechanisms for ejecting the ink drops from a predetermined distance between the writing mechanisms and a printing surface of the print medium at a print zone of a hard copy apparatus. The process includes the steps of: heating sequentially received sheets of the print medium such that the printing surface is higher than ambient atmospheric temperature; and providing a laminar flow of air substantially continuously across the printing surface of the sheet through the print zone.
In another basic aspect, the present invention provides an ink-jet hard copy apparatus, having a sheet media input supply and including: a paper transport for sequentially selecting a sheet of print medium from the input supply and transporting the sheet through a print zone region of the apparatus where drops of ink are deposited on a printing surface of the sheet; at least one ink-jet writing instrument for scanning the print zone substantially perpendicularly to direction of transporting the sheet and selectively ejecting drops of ink onto the printing surface, the drops having a predetermined flight time from the instrument to the printing surface; at least one heater mounted with respect to the print zone region for imparting thermal energy to the printing surface such that drying time of drops once deposited on the printing surface is reduced; and at least one airflow device for generating a laminar flow of air through the print zone region and across the printing surface such that drying time of drops once deposited on the printing surface is reduced further from a drying time produced by the heater alone.
In another basic aspect, the present invention provides a scanning ink-jet pen for a hard copy apparatus having a mechanism for producing an air flow through a print zone. The pen includes: printhead mechanisms for firing ink drops from the pen to a surface of adjacently positioned print media, the ink drops having a predetermined flight time between the printhead mechanisms and the surface; and an air flow deflector mounted such that the air flow is interrupted and substantially as prevented from crossing the print zone during the predetermined flight time.
Some advantages of the present invention are:
it produces more favorable air flow patterns across an ink-jet printer print zone;
less active heating is required;
in vacuum platen type apparatus, less vacuum is required;
any ink drop flight errors caused by air flow through the ink zone are more uniformly distributed and easier to compensate;
air flow patterns produced more closely to the print zone;
a more compact commercial product design is enabled;
ink drop flight is protected from interfering air flow patterns;
paper cockle is reduced by improved drying time;
it provides additional cooling for ink-jet printhead mechanisms; and
it provides a system which drives residual vapor away from the print zone and to a location where it can be captured and filtered.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01 (d) merely to apprize the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.